Please help, shimano xt m772 broken bolt spring?

Hi all, wondered if anyone can help, was changing gear today and felt a faint snap/sheer noise, my gears don't work properly now and the chain has gone really slack when in the bottom cog on the back.

I can't see anything broken but when I took the mech off there was a small half round size black washer, looks like it's only half of what should be there? My question is does anyone else have this mech and if so is there some kind of tiny round torsion spring inside the bolt that screws in to the frame mech hanger?

Really hope this makes sense... Oh and if it helps the bolt just falls out the mech body hole where as others I see the bolt is solid and just spins round inside the mech body.

My guess is the half black washer is the circlip that holds the shaped silver washer in place. The shaped washer holds the return spring inside the mech body, and has a lug that rests (under tension when you wind the spring around) against a raised stop on the mech body. Before you bother trying to repair it, check the stop isn't damaged, if it's virtually gone then it's probably new mech time, partly damaged and you may bend the lug a little and find it'll locate.